This invention relates to implantation of phosphorus ions into silicon. It more particularly relates to an improved low dose phosphorus implantation method for &lt;100&gt; P-type silicon crystal faces, to permit anneal in a strongly oxidizing atmosphere for PN junction passivation, without concurrently inducing PN junction leakage.
A P-type surface portion of a monocrystalline silicon body can be doped to N-type conductivity by exposing the surface portion to a beam of phosphorus ions and then annealing the silicon body. The resultant N-type region forms a PN junction with unimplanted adjacent portions of the P-type silicon. In such annealing, the silicon body is heated to repair defects in the silicon crystal lattice caused by the implantation and move implanted phosphorus atoms to substitutional sites in the lattice. Annealing can be done in reducing, inert or oxidizing atmospheres. If done in a strongly oxidizing atmosphere, i.e., moist oxygen, the anneal will also produce a passivating silicon dioxide coating on the silicon surface over the PN junction.
Phosphorus implants greater than about 5 .times. 10.sup.15 phosphorus ions per square centimeter in &lt;100&gt; P-type silicon can produce low leakage PN junctions when annealed in any suitable atmosphere, including moist oxygen. This is also true for phosphorus implants of any dose in &lt;111&gt; P-type silicon. Hence, if desired, the PN junction can be simultaneously oxide passivated during implantation anneal. This is particularly advantageous in the manufacture of integrated circuits. However, we have recognized that lower dose phosphorus implants in &lt;100&gt; silicon do not produce PN junctions of low leakage upon anneal in a strongly oxidizing atmosphere. Until now, if oxide passivation of diodes from such low dose phosphorus implants were desired, it had to be done in an extra step after anneal. Otherwise, one had to be content with a higher PN junction leakage.
Analogous differences between boron implants in &lt;111&gt; and &lt;100&gt; silicon are described in our earlier United States patent application Ser. No. 759,903, entitled "Improved Method of Boron Implantation for Oxide Passivated Diodes in &lt;100&gt; N-Type Silicon", filed Jan. 17, 1977 and assigned to the assignee of this invention. In our prior U.S. Ser. No. 759,903 we disclose adding a severe damage implant to boron implants in &lt;100&gt; silicon to obtain low leakage diodes from strongly oxidizing anneals. We have now found that with damage implants from certain selected ions, this effect can also be obtained from low dose phosphorus implants in &lt;100&gt; P-type silicon. Thus, diodes from low dose phosphorus implants in &lt;100&gt; silicon can now be oxide passivated during implantation anneal without also incurring the usual high PN junction leakage.